Some color image sensing apparatus produce a sampled color image signal by sampling one color component of an image at a higher spatial sampling frequency than other color components. The more highly sampled colored component usually corresponds to the portion of the spectrum to which the human eye is most sensitive to image detail, and is often called the luminance component. The term "luminance component" as used herein refers to the most highly sampled color component, whether it be green, white, or some other color.
The less highly sampled color components, are often referred to as chrominance components and may comprise, for example, red and blue, or cyan and yellow. The term "chrominance component" as used herein refers to a less highly sampled color component of an image regardless of the particular color.
In some color image sensing apparatus of this type, one image sensor samples the luminance component of the image, and another image sensor samples two chrominance components. The image sensing elements on the image sensor that samples the chrominance components alternate between one color and the other.
In other color image sensing apparatus, a single image sensor is employed to sense the luminance and two chrominance components of the image. The image sensing elements of the image sensor alternate between elements for sampling luminance and elements for sampling one and then the other of the chrominance components.
When reconstructing an image from the sampled color image signals produced by these color image sensors, values of the luminance and chrominance components are provided for each sample location. This is generally accomplished by using some form of linear interpolation between sample values of the less highly sampled color components. Reconstruction of the color image by linear interpolation of the chrominance components results in the appearance of colored fringes in areas of image detail, due to a deviation of the interpolated values from the actual color values existing in the original image.
It is known to employ signal processing to reduce the appearance of these colored fringes in the reconstructed images. In U.S. Pat. No. 4,176,373, issued Nov. 27, 1979, Dillon and Bayer disclose analog signal processing apparatus for interpolating values between the sampled luminance and chrominance values in a sampled color image signal produced by a single image sensor having a checkerboard pattern of luminance (green) image sensing elements interspersed with chrominance component (red and blue) image sensing elements. The apparatus performs a linear interpolation between chrominance component samples, then adds a high spatial frequency portion of the luminance signal to the interpolated chrominance component signals. Adding the high spatial frequency portion of the luminance signal to the interpolated chrominance component signals reduces the appearance of color fringes in areas of image detail by driving the high spatial frequency portion of the image signal towards a neutral color. Although the signal processing disclosed by Dillon and Bayer is effective to reduce the appearance of color fringing in the detailed areas of the image, the method achieves this object at the expense of altering the sampled values of the chrominance component signals of the image signal. As a result, some hue shifts are introduced in areas of image detail even as the appearance of color fringing is reduced.
The object of the present invention is to provide a signal processing method for reducing the appearance of colored fringes in areas of fine image detail without introducing unwanted hue shifts.